[ntp:questions] SIRF time output wobble, GPS or NTP fault?

Dave Hart davehart at gmail.com
Wed Mar 7 19:04:33 UTC 2012

On Wed, Mar 7, 2012 at 09:25, David J Taylor
<david-taylor at blueyonder.co.uk.invalid> wrote:
> There is a question about the Windows port of NTP which I don't have the
> answer to.  Dave Hart has put a lot of work into the Windows port, both in
> the serial data handling, and the serial port driver itself.  What I don't
> know is whether his enhancement to the serial port time-stamping also apply
> to the virtual COM port drivers such as would be used for NMEA over USB.  I
> expect they would, though.

No, the PPSAPI support is in serialpps.sys, a slightly-modified 16550
UART driver.  When using serial-over-USB, a different driver (for the
pl2003 or similar chip) presents the serial port and it will not have
the PPSAPI ioctl to timestamp DCD in the driver.

On Windows only, ntpd has a hack user-mode PPS timestamping
implemented in the serial I/O layer which substitutes the last PPS
timestamp taken by ntpd after Windows indicates the DCD transition for
the end-of-line serial timestamp of the first line received after each
PPS event.  That requires configuring ntpd to use only the first
sentence after the PPS, and it won't work on Linux, so I doubt Ron
will be interested in that approach.

The Linux PPSAPI has the same type of driver issue -- the code to
timestamp DCD is in the UART driver, but not the pl2003 driver, so
PPSAPI will work with a real serial port but not with a USB/serial

Dave Hart

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